Stability of Carbonate Rocks Containing Acid Wormholes Under High Confining Pressures

Stimulating carbonate reservoirs by injecting acids results in channels within the rock called wormholes. This study reveals the stability of wormholed chalk and limestone samples under high confining pressure using a triaxial cell. The experiments were conducted on four Austin chalk (AC) and four Indiana limestone (IL) core samples (1.5 inches x 3 inches). The samples were characterized by their petrophysical (i.e., porosity and permeability) and elastic properties (i.e., Young's modulus). Two of each rock type were acidized to generate wormholes using a core flooding setup. Then the samples were subjected to a high confining pressure of up to 88 MPa using a triaxial/acoustic cell. In addition, the permeability of the samples under increasing confining pressure was reported. Medical CT scan images were taken for the wormholed samples before and after subjecting them to high confining pressures. The CT scan images and rock properties were imported to COMSOL finite element to simulate the deformation under high stresses. The results revealed that wormholes closed under confining in the chalk samples while it was mechanically stable in the IL samples. This was attributed to the rigidity of limestone as compared to weak chalk samples. Fractures were observed in one of the chalk samples, while no fractures were noticed in the limestone. The finite element simulations confirmed the experimental outcomes and showed that the wormhole branches were more susceptible to deformation and closure. Wormholes in weak carbonate rocks are subject to closure under high confining pressuresWormhole branches tend to close faster due to the higher stress concentrationPermeability of the wormhole is a strong function of the confining pressure